In its over 35 years of existence, the musical “Little Shop of Horrors” has grown from being an oddity as an off-beat but fun musical, to its now classic status. It’s not just that it is based on a 1960 low budget horror flick, or that it revolves around a nerd flower-shop worker named Seymour, his tawdry but good-hearted girl, Audrey, and the man-eating plant from outer space. The most surprising aspect of “Little Shop” is that just barely beneath the surface beats a sentimental heart. It never condescends to its characters or reduces them to cartoons.